Teacher notes: Detection of DNA by gel electrophoresis
What the lesson is about
This resource can be used as a virtual teaching tool to highlight the use of gel
electrophoresis to detect and measure DNA fragments.
The process works by loading a DNA sample mixed with dye onto an agarose
gel immersed in buffer. When a current is connected, the negatively charged
DNA is pulled through the agarose towards the positive pole. The smaller the
fragment of DNA, the faster it will travel through the gel, allowing separation
of fragments of different sizes. The gel is stained and destained to clearly
reveal the bands, which can then be gauged against marker DNA (of
predetermined size).
Movies should be opened in QuickTime and can be controlled by clicking
forward and back arrow keys.
Learning outcomes
Higher Biology (revised), Unit 1
1. The structure and replication of DNA b) (ii) Polymerase chain reaction
Aims
Students will be able to describe the process of gel electrophoresis as a
technique to measure the size of DNA fragments.
Slides
Detection of DNA by gel electrophoresis – title slide
(Click) Shows an agarose gel with wells. Samples containing DNA are mixed
with a blue dye and loaded into the wells with a micro pipette. ( Four wells
will be loaded.)
(Click) The gel is placed in a buffer and connected to an electric current. As
DNA is negatively charged it is drawn through the gel towards the positive
charge (anode).
(Click) The gel is rotated to show that different -sized fragments of DNA
move at different rates. Larger fragments move slower and are seen at the top
of the gel, whilst smaller fragments move faster.
(Click) The far right-hand well shows a DNA ladder or marker DNA. This
contains DNA fragments of predefined sizes (ranging from 100 to 600 bas e
TEACHER’S NOTES: DETECTION OF DNA BY GEL ELECTROPHORESIS (H, BIOLOGY)
© Learning and Teaching Scotland 2011
1
pairs (bp)). This allows us to estimate the size of the different fragments to
see if they are as expected.
The bands on this gel (from left to right) are approximately 500 base pairs,
240 base pairs, larger than 600 base pairs.
Throughout the lesson you may want to prompt students by asking them to
describe what is happening in the animation. Written answer s may also be
sought:
1.
Why do different sizes of DNA give different bands?
Larger fragments of DNA are caught up in the gel more easily and take
longer to pass through it; smaller fragments of DNA pass through the
gel more easily and therefore more quickly – a bit like a sieve. In this
way, electrophoresis separates DNA fragments on the basis of size.
Within a given time, smaller fragments of DNA will travel further
through the gel than larger fragments. Many DNA fragments of the
same length will reach the same point in the gel , where they will appear
as a band.
2.
Describe the role of the DNA ladder.
Gel electrophoresis separates DNA fragments on th e basis of relative
size. The DNA ladder contains DNA fragments of known length. It can
therefore act as a ‘ruler’ to show the approximate length of DNA
fragment within each band.
2
TEACHER’S NOTES: DETECTION OF DNA BY GEL ELECTROPHORESIS (H, BIOLOGY)
© Learning and Teaching Scotland 2011